The oil and gas industry has known for some time that vibrating a casing during primary cementing can improve cementing of the casing to the wellbore. Vibrating the casing will drive out air pockets and break the gel strength of the cement slurry used in primary cementing which in turn helps maintain the slurry's pressure above the pressure of fluids in the surrounding formation. Breaking the cement slurry's gel strength and driving out its air pockets are believed, therefore, to prevent a phenomenon known as annular fluid flow. Annular fluid flow occurs where a gas or liquid from one formation flows to the surface or to another formation through the cemented annulus before the cement develops sufficient strength to prevent such fluid flow.
Annular fluid flow leads to either an increased permeability of or surface channel formation on the cement sheath which fixes the casing in the wellbore. Such increased permeability or surface channels provide a means for fluid communication between a zone containing hydrocarbons and a zone containing nonhydrocarbon fluids in the formations surrounding the wellbore. Both of these fluid communication pathways permit nonhydrocarbon fluids to flow to the casing/cement sheath perforations which provide the passages for producing hydrocarbons into the casing borehole. Both pathways may also permit hydrocarbon fluids to flow in and contaminate adjacent water sands. Therefore, the cement sheath's ability to prevent mixing of nonhydrocarbon fluids with produced hydrocarbons is significantly diminished as a result of annular fluid flow.
A more detailed discussion and examples of annular fluid flow are provided in U.S. Pat. No. 4,407,365 issued to C. E. Cooke, Jr., Oct. 4, 1983 and in a Society of Petroleum Engineers ("SPE") paper, SPE 14199, by C. E. Cooke, O. J. Gonzalez, and D. J. Broussard, entitled Primary Cementing Improvement by Casing Vibration During Cement Curing Time, presented at the 1985 Annual Technical Conference, Las Vegas, Nev., September 22-25.
As a pipe or casing suspended in a liquid is vibrated, various types of vibrational modes may be established such as tube waves, extensional waves, torsional waves, flexural waves, and string waves. In the following discussion, only tube waves and extensional waves will be discussed since those are the types of vibrational modes most capable of transmitting energy downhole.
Extensional waves are produced in the solid body of the casing when energy in the form of a mechanical force is transferred to the casing substantially parallel to its longitudinal axis. Extensional waves are damped by liquid-solid and/or solid-solid frictional forces. Liquid-solid friction damps extensional waves as liquid drags on the casing wall. Solid-solid friction damps extensional waves as the outside of the casing wall contacts the borehole wall. An extensional wave's travel distance, therefore, is dictated by the magnitude of the energy transferred to the casing and the damping forces acting on it. As a result of such damping forces, extensional waves in well casings typically have a limited travel distance, which is generally on the order of a thousand feet or less.
Tube waves are propagated through a liquid contained in the casing when a pressure pulse is injected into the liquid. Tube waves transmit most of their energy through the liquid and therefore are not affected by friction of the casing on the borehole wall. Tube waves are damped to a certain extent by liquid-solid frictional forces, but generally, their damping affect on tube waves is only slight. Therefore, tube waves can travel substantial distances through a liquid filled casing with little attenuation to their amplitude.
In their SPE publication Cooke et al. suggested vibrating the well casing by pushing and pulling on the end of the pipe at the wellhead. This method proved to be impractical because the extensional waves thereby produced were attenuated too rapidly to produce any significant vibration at depths in excess of about 1000 feet. Other methods of vibrating casing disclosed in Cooke's U.S. Pat. No. 4,407,365 include using intermittent explosive charges to cause pressure pulses, explosive charges to propel a projectile against the casing wall, or hydraulic jars or electrical, mechanical, or hydraulic vibrators to directly vibrate the casing wall. Some of these vibration techniques can be employed to deliver their vibrating force at preselected depths by lowering the charge or vibrator on a wireline. However, such a procedure would add significant time and expense to the cementing process.
Therefore, a need exists for an efficient, safe, and cost-effective method for inducing significant downhole vibration at depths of about 1000 feet or more. More specifically, a need exists for an improved method for vibrating a well casing at depths of about 1000 feet or more during primary cementing so that annular fluid flow may be prevented, particularly in regions near the hydrocarbon producing zone of the formation.